markdownlint
A Node.js style checker and lint tool for Markdown/CommonMark files.
Install
npm install markdownlint --save-devOverview
The Markdown markup language is designed to be easy to read, write, and understand. It succeeds - and its flexibility is both a benefit and a drawback. Many styles are possible, so formatting can be inconsistent. Some constructs don't work well in all parsers and should be avoided. The CommonMark specification standardizes parsers - but not authors.
markdownlint is a
static analysis
tool for Node.js with a library of rules
to enforce standards and consistency for Markdown files. It was
inspired by - and heavily influenced by - Mark Harrison's
markdownlint for
Ruby. The initial rules, rule documentation,
and test cases came directly from that project.
Related
- CLI
- markdownlint-cli command-line interface for Node.js (works with pre-commit)
- markdownlint-cli2 command-line interface for Node.js (works with pre-commit)
- GitHub
- Editor
- Tooling
- Ruby
References
The following specifications are considered authoritative in cases of ambiguity:
Demonstration
markdownlint demo, an interactive, in-browser
playground for learning and exploring.
Rules / Aliases
- MD001 heading-increment - Heading levels should only increment by one level at a time
- MD003 heading-style - Heading style
- MD004 ul-style - Unordered list style
- MD005 list-indent - Inconsistent indentation for list items at the same level
- MD007 ul-indent - Unordered list indentation
- MD009 no-trailing-spaces - Trailing spaces
- MD010 no-hard-tabs - Hard tabs
- MD011 no-reversed-links - Reversed link syntax
- MD012 no-multiple-blanks - Multiple consecutive blank lines
- MD013 line-length - Line length
- MD014 commands-show-output - Dollar signs used before commands without showing output
- MD018 no-missing-space-atx - No space after hash on atx style heading
- MD019 no-multiple-space-atx - Multiple spaces after hash on atx style heading
- MD020 no-missing-space-closed-atx - No space inside hashes on closed atx style heading
- MD021 no-multiple-space-closed-atx - Multiple spaces inside hashes on closed atx style heading
- MD022 blanks-around-headings - Headings should be surrounded by blank lines
- MD023 heading-start-left - Headings must start at the beginning of the line
- MD024 no-duplicate-heading - Multiple headings with the same content
- MD025 single-title/single-h1 - Multiple top-level headings in the same document
- MD026 no-trailing-punctuation - Trailing punctuation in heading
- MD027 no-multiple-space-blockquote - Multiple spaces after blockquote symbol
- MD028 no-blanks-blockquote - Blank line inside blockquote
- MD029 ol-prefix - Ordered list item prefix
- MD030 list-marker-space - Spaces after list markers
- MD031 blanks-around-fences - Fenced code blocks should be surrounded by blank lines
- MD032 blanks-around-lists - Lists should be surrounded by blank lines
- MD033 no-inline-html - Inline HTML
- MD034 no-bare-urls - Bare URL used
- MD035 hr-style - Horizontal rule style
- MD036 no-emphasis-as-heading - Emphasis used instead of a heading
- MD037 no-space-in-emphasis - Spaces inside emphasis markers
- MD038 no-space-in-code - Spaces inside code span elements
- MD039 no-space-in-links - Spaces inside link text
- MD040 fenced-code-language - Fenced code blocks should have a language specified
- MD041 first-line-heading/first-line-h1 - First line in a file should be a top-level heading
- MD042 no-empty-links - No empty links
- MD043 required-headings - Required heading structure
- MD044 proper-names - Proper names should have the correct capitalization
- MD045 no-alt-text - Images should have alternate text (alt text)
- MD046 code-block-style - Code block style
- MD047 single-trailing-newline - Files should end with a single newline character
- MD048 code-fence-style - Code fence style
- MD049 emphasis-style - Emphasis style
- MD050 strong-style - Strong style
- MD051 link-fragments - Link fragments should be valid
- MD052 reference-links-images - Reference links and images should use a label that is defined
- MD053 link-image-reference-definitions - Link and image reference definitions should be needed
- MD054 link-image-style - Link and image style
- MD055 table-pipe-style - Table pipe style
- MD056 table-column-count - Table column count
See Rules.md for more details.
Custom Rules
In addition to built-in rules, custom rules can be used to address
project-specific requirements. To find community-developed rules use
keyword markdownlint-rule on npm.
To implement your own rules, refer to CustomRules.md.
Tags
Tags group related rules and can be used to enable/disable multiple rules at once.
accessibility-MD045atx-MD018,MD019atx_closed-MD020,MD021blank_lines-MD012,MD022,MD031,MD032,MD047blockquote-MD027,MD028bullet-MD004,MD005,MD007,MD032code-MD014,MD031,MD038,MD040,MD046,MD048emphasis-MD036,MD037,MD049,MD050hard_tab-MD010headings-MD001,MD003,MD018,MD019,MD020,MD021,MD022,MD023,MD024,MD025,MD026,MD036,MD041,MD043hr-MD035html-MD033images-MD045,MD052,MD053,MD054indentation-MD005,MD007,MD027language-MD040line_length-MD013links-MD011,MD034,MD039,MD042,MD051,MD052,MD053,MD054ol-MD029,MD030,MD032spaces-MD018,MD019,MD020,MD021,MD023spelling-MD044table-MD055,MD056ul-MD004,MD005,MD007,MD030,MD032url-MD034whitespace-MD009,MD010,MD012,MD027,MD028,MD030,MD037,MD038,MD039
Configuration
Text passed to markdownlint is parsed as Markdown, analyzed, and any
issues reported. Two kinds of text are ignored by most rules:
- HTML comments
- Front matter (see
options.frontMatterbelow)
Rules can be enabled, disabled, and configured via options.config
(described below) to define the expected behavior for a set of inputs.
To enable or disable rules at a particular location within a file, add
one of these markers to the appropriate place (HTML comments don't
appear in the final markup):
- Disable all rules:
<!-- markdownlint-disable --> - Enable all rules:
<!-- markdownlint-enable --> - Disable all rules for the current line:
<!-- markdownlint-disable-line --> - Disable all rules for the next line:
<!-- markdownlint-disable-next-line --> - Disable one or more rules by name:
<!-- markdownlint-disable MD001 MD005 --> - Enable one or more rules by name:
<!-- markdownlint-enable MD001 MD005 --> - Disable one or more rules by name for the current line:
<!-- markdownlint-disable-line MD001 MD005 --> - Disable one or more rules by name for the next line:
<!-- markdownlint-disable-next-line MD001 MD005 --> - Capture the current rule configuration:
<!-- markdownlint-capture --> - Restore the captured rule configuration:
<!-- markdownlint-restore -->
For example:
<!-- markdownlint-disable-next-line no-space-in-emphasis -->
space * in * emphasisOr:
space * in * emphasis <!-- markdownlint-disable-line no-space-in-emphasis -->Or:
<!-- markdownlint-disable no-space-in-emphasis -->
space * in * emphasis
<!-- markdownlint-enable no-space-in-emphasis -->To temporarily disable rule(s), then restore the former configuration:
<!-- markdownlint-capture -->
<!-- markdownlint-disable -->
any violations you want
<!-- markdownlint-restore -->The initial configuration is captured by default (as if every document
began with <!-- markdownlint-capture -->), so the pattern above can
be expressed more simply:
<!-- markdownlint-disable -->
any violations you want
<!-- markdownlint-restore -->Changes take effect starting with the line a comment is on, so the following has no effect:
space * in * emphasis <!-- markdownlint-disable --> <!-- markdownlint-enable -->To apply changes to an entire file regardless of where the comment is located, the following syntax is supported:
- Disable all rules:
<!-- markdownlint-disable-file --> - Enable all rules:
<!-- markdownlint-enable-file --> - Disable one or more rules by name:
<!-- markdownlint-disable-file MD001 --> - Enable one or more rules by name:
<!-- markdownlint-enable-file MD001 -->
This can be used to "hide" markdownlint comments at the bottom of a file.
In cases where it is desirable to change the configuration of one or more rules for a file, the following more advanced syntax is supported:
- Configure:
<!-- markdownlint-configure-file { options.config JSON } -->
For example:
<!-- markdownlint-configure-file { "hr-style": { "style": "---" } } -->or
<!-- markdownlint-configure-file
{
"hr-style": {
"style": "---"
},
"no-trailing-spaces": false
}
-->These changes apply to the entire file regardless of where the comment is
located. Multiple such comments (if present) are applied top-to-bottom. By
default, content of markdownlint-configure-file is assumed to be JSON, but
options.configParsers can be used to support
alternate formats.
API
Linting
Standard asynchronous API:
/**
* Lint specified Markdown files.
*
* @param {Options} options Configuration options.
* @param {LintCallback} callback Callback (err, result) function.
* @returns {void}
*/
function markdownlint(options, callback) { ... }Synchronous API (for build scripts, etc.):
/**
* Lint specified Markdown files synchronously.
*
* @param {Options} options Configuration options.
* @returns {LintResults} Results object.
*/
function markdownlint.sync(options) { ... }Promise API (in the promises namespace like Node.js's
fs Promises API):
/**
* Lint specified Markdown files.
*
* @param {Options} options Configuration options.
* @returns {Promise<LintResults>} Results object.
*/
function markdownlint(options) { ... }options
Type: Object
Configures the function. All properties are optional, but at least one
of files or strings should be set to provide input.
options.config
Type: Object mapping String to Boolean | Object
Configures the rules to use.
Object keys are rule names or aliases and values are the rule's configuration.
The value false disables a rule, true enables its default configuration,
and passing an object customizes its settings. Setting the special default
rule to true or false includes/excludes all rules by default. Enabling
or disabling a tag name (ex: whitespace) affects all rules having that
tag.
The default rule is applied first, then keys are processed in order
from top to bottom with later values overriding earlier ones. Keys
(including rule names, aliases, tags, and default) are not case-sensitive.
Example:
{
"default": true,
"MD003": { "style": "atx_closed" },
"MD007": { "indent": 4 },
"no-hard-tabs": false,
"whitespace": false
}See .markdownlint.jsonc and/or .markdownlint.yaml for an example configuration object with all properties set to the default value.
Sets of rules (known as a "style") can be stored separately and loaded as JSON.
Example of referencing a built-in style from JavaScript:
const options = {
"files": [ "..." ],
"config": require("style/relaxed.json")
};Example doing so from .markdownlint.json via extends (more on this below):
{
"extends": "markdownlint/style/relaxed"
}See the style directory for more samples.
See markdownlint-config-schema.json
for the JSON Schema of the options.config
object.
See ValidatingConfiguration.md for ways to use the JSON Schema to validate configuration.
For more advanced scenarios, styles can reference and extend other styles.
The readConfig and readConfigSync functions can be used to read such
styles.
For example, assuming a base.json configuration file:
{
"default": true
}And a custom.json configuration file:
{
"extends": "base.json",
"line-length": false
}Then code like the following:
const options = {
"config": markdownlint.readConfigSync("./custom.json")
};Merges custom.json and base.json and is equivalent to:
const options = {
"config": {
"default": true,
"line-length": false
}
};options.configParsers
Type: Optional Array of Function taking (String) and returning Object
Array of functions to parse the content of markdownlint-configure-file blocks.
As shown in the Configuration section, inline comments can be
used to customize the configuration object for a document. By
default, the JSON.parse built-in is used, but custom parsers can be specified.
Content is passed to each parser function until one returns a value (vs.
throwing an exception). As such, strict parsers should come before flexible
ones.
For example:
[ JSON.parse, require("toml").parse, require("js-yaml").load ]options.customRules
Type: Array of Object
List of custom rules to include with the default rule set for linting.
Each array element should define a rule. Rules are typically exported by another package, but can be defined locally.
Example:
const extraRules = require("extraRules");
const options = {
"customRules": [ extraRules.one, extraRules.two ]
};See CustomRules.md for details about authoring custom rules.
options.files
Type: Array of String
List of files to lint.
Each array element should be a single file (via relative or absolute path); globbing is the caller's responsibility.
Example: [ "one.md", "dir/two.md" ]
options.frontMatter
Type: RegExp
Matches any front matter found at the beginning of a file.
Some Markdown content begins with metadata; the default RegExp for
this option ignores common forms of "front matter". To match differently,
specify a custom RegExp or use the value null to disable the feature.
The default value:
/((^---\s*$[\s\S]+?^---\s*)|(^\+\+\+\s*$[\s\S]+?^(\+\+\+|\.\.\.)\s*)|(^\{\s*$[\s\S]+?^\}\s*))(\r\n|\r|\n|$)/mIgnores YAML, TOML, and JSON front matter such as:
---
layout: post
title: Title
---
Note: Matches must occur at the start of the file.
options.fs
Type: Object implementing the file system API
In advanced scenarios, it may be desirable to bypass the default file system
API. If a custom file system implementation is provided, markdownlint will use
that instead of invoking require("fs").
Note: The only methods called are readFile and readFileSync.
options.handleRuleFailures
Type: Boolean
Catches exceptions thrown during rule processing and reports the problem as a rule violation.
By default, exceptions thrown by rules (or the library itself) are unhandled
and bubble up the stack to the caller in the conventional manner. By setting
handleRuleFailures to true, exceptions thrown by failing rules will
be handled by the library and the exception message logged as a rule violation.
This setting can be useful in the presence of (custom) rules that encounter
unexpected syntax and fail. By enabling this option, the linting process
is allowed to continue and report any violations that were found.
options.markdownItPlugins
Type: Array of Array of Function and plugin parameters
Specifies additional markdown-it plugins to use when parsing input. Plugins can be used to support additional syntax and features for advanced scenarios.
Each item in the top-level Array should be of the form:
[ require("markdown-it-plugin"), plugin_param_0, plugin_param_1, ... ]options.noInlineConfig
Type: Boolean
Disables the use of HTML comments like <!-- markdownlint-enable --> to toggle
rules within the body of Markdown content.
By default, properly-formatted inline comments can be used to create exceptions
for parts of a document. Setting noInlineConfig to true ignores all such
comments.
options.resultVersion
Type: Number
Specifies which version of the result object to return (see the "Usage"
section below for examples).
Passing a resultVersion of 0 corresponds to the original, simple format
where each error is identified by rule name and line number. This is
deprecated.
Passing a resultVersion of 1 corresponds to a detailed format where each
error includes information about the line number, rule name, alias, description,
as well as any additional detail or context that is available. This is
deprecated.
Passing a resultVersion of 2 corresponds to a detailed format where each
error includes information about the line number, rule names, description, as
well as any additional detail or context that is available. This is
deprecated.
Passing a resultVersion of 3 corresponds to the detailed version 2 format
with additional information about how to fix automatically-fixable errors. In
this mode, all errors that occur on each line are reported (other versions
report only the first error for each rule). This is the default.
options.strings
Type: Object mapping String to String
Map of identifiers to strings for linting.
When Markdown content is not available as files, it can be passed as
strings. The keys of the strings object are used to identify each
input value in the result summary.
Example:
{
"readme": "# README\n...",
"changelog": "# CHANGELOG\n..."
}callback
Type: Function taking (Error, Object)
Standard completion callback.
result
Type: Object
Call result.toString() for convenience or see below for an example of the
structure of the result object. Passing the value true to toString()
uses rule aliases (ex: no-hard-tabs) instead of names (ex: MD010).
Config
The options.config configuration object is simple and can be stored in a file
for readability and easy reuse. The readConfig and readConfigSync functions
load configuration settings and support the extends keyword for referencing
other files (see above).
By default, configuration files are parsed as JSON (and named
.markdownlint.json). Custom parsers can be provided to handle other formats
like JSONC, YAML, and TOML.
Asynchronous API:
/**
* Read specified configuration file.
*
* @param {string} file Configuration file name.
* @param {ConfigurationParser[] | ReadConfigCallback} parsers Parsing function.
* @param {Object} [fs] File system implementation.
* @param {ReadConfigCallback} [callback] Callback (err, result) function.
* @returns {void}
*/
function readConfig(file, parsers, fs, callback) { ... }Synchronous API:
/**
* Read specified configuration file synchronously.
*
* @param {string} file Configuration file name.
* @param {ConfigurationParser[]} [parsers] Parsing function(s).
* @param {Object} [fs] File system implementation.
* @returns {Configuration} Configuration object.
*/
function readConfigSync(file, parsers, fs) { ... }Promise API (in the promises namespace like Node.js's
fs Promises API):
/**
* Read specified configuration file.
*
* @param {string} file Configuration file name.
* @param {ConfigurationParser[]} [parsers] Parsing function(s).
* @param {Object} [fs] File system implementation.
* @returns {Promise<Configuration>} Configuration object.
*/
function readConfig(file, parsers, fs) { ... }file
Type: String
Location of configuration file to read.
The file is resolved relative to the current working directory. If an
extends key is present once read, its value will be resolved as a path
relative to file and loaded recursively. Settings from a file referenced by
extends are applied first, then those of file are applied on top (overriding
any of the same keys appearing in the referenced file). If either the file or
extends path begins with the ~ directory, it will act as a placeholder for
the home directory.
parsers
Type: Optional Array of Function taking (String) and returning Object
Array of functions to parse configuration files.
The contents of a configuration file are passed to each parser function until one of them returns a value (vs. throwing an exception). Consequently, strict parsers should come before flexible parsers.
For example:
[ JSON.parse, require("toml").parse, require("js-yaml").load ]fs
Type: Optional Object implementing the file system API
In advanced scenarios, it may be desirable to bypass the default file system
API. If a custom file system implementation is provided, markdownlint will use
that instead of invoking require("fs").
Note: The only methods called are readFile, readFileSync, access, and
accessSync.
callback
Type: Function taking (Error, Object)
Standard completion callback.
result
Type: Object
Configuration object.
Usage
Invoke markdownlint and use the result object's toString method:
const markdownlint = require("markdownlint");
const options = {
"files": [ "good.md", "bad.md" ],
"strings": {
"good.string": "# good.string\n\nThis string passes all rules.",
"bad.string": "#bad.string\n\n#This string fails\tsome rules."
}
};
markdownlint(options, function callback(err, result) {
if (!err) {
console.log(result.toString());
}
});Output:
bad.string: 3: MD010/no-hard-tabs Hard tabs [Column: 19]
bad.string: 1: MD018/no-missing-space-atx No space after hash on atx style heading [Context: "#bad.string"]
bad.string: 3: MD018/no-missing-space-atx No space after hash on atx style heading [Context: "#This string fails some rules."]
bad.string: 1: MD041/first-line-heading/first-line-h1 First line in a file should be a top-level heading [Context: "#bad.string"]
bad.md: 3: MD010/no-hard-tabs Hard tabs [Column: 17]
bad.md: 1: MD018/no-missing-space-atx No space after hash on atx style heading [Context: "#bad.md"]
bad.md: 3: MD018/no-missing-space-atx No space after hash on atx style heading [Context: "#This file fails some rules."]
bad.md: 1: MD041/first-line-heading/first-line-h1 First line in a file should be a top-level heading [Context: "#bad.md"]
Or invoke markdownlint.sync for a synchronous call:
const result = markdownlint.sync(options);
console.log(result.toString());To examine the result object directly:
markdownlint(options, function callback(err, result) {
if (!err) {
console.dir(result, { "colors": true, "depth": null });
}
});Output:
{
"good.md": [],
"bad.md": [
{ "lineNumber": 3,
"ruleNames": [ "MD010", "no-hard-tabs" ],
"ruleDescription": "Hard tabs",
"ruleInformation": "https://github.com/DavidAnson/markdownlint/blob/v0.0.0/doc/md010.md",
"errorDetail": "Column: 17",
"errorContext": null,
"errorRange": [ 17, 1 ] },
{ "lineNumber": 1,
"ruleNames": [ "MD018", "no-missing-space-atx" ],
"ruleDescription": "No space after hash on atx style heading",
"ruleInformation": "https://github.com/DavidAnson/markdownlint/blob/v0.0.0/doc/md018.md",
"errorDetail": null,
"errorContext": "#bad.md",
"errorRange": [ 1, 2 ] },
{ "lineNumber": 3,
"ruleNames": [ "MD018", "no-missing-space-atx" ],
"ruleDescription": "No space after hash on atx style heading",
"ruleInformation": "https://github.com/DavidAnson/markdownlint/blob/v0.0.0/doc/md018.md",
"errorDetail": null,
"errorContext": "#This file fails\tsome rules.",
"errorRange": [ 1, 2 ] },
{ "lineNumber": 1,
"ruleNames": [ "MD041", "first-line-heading", "first-line-h1" ],
"ruleDescription": "First line in a file should be a top-level heading",
"ruleInformation": "https://github.com/DavidAnson/markdownlint/blob/v0.0.0/doc/md041.md",
"errorDetail": null,
"errorContext": "#bad.md",
"errorRange": null }
]
}Integration with the gulp build system is straightforward:
const gulp = require("gulp");
const through2 = require("through2");
const markdownlint = require("markdownlint");
gulp.task("markdownlint", function task() {
return gulp.src("*.md", { "read": false })
.pipe(through2.obj(function obj(file, enc, next) {
markdownlint(
{ "files": [ file.relative ] },
function callback(err, result) {
const resultString = (result || "").toString();
if (resultString) {
console.log(resultString);
}
next(err, file);
});
}));
});Output:
[00:00:00] Starting 'markdownlint'...
bad.md: 3: MD010/no-hard-tabs Hard tabs [Column: 17]
bad.md: 1: MD018/no-missing-space-atx No space after hash on atx style heading [Context: "#bad.md"]
bad.md: 3: MD018/no-missing-space-atx No space after hash on atx style heading [Context: "#This file fails some rules."]
bad.md: 1: MD041/first-line-heading/first-line-h1 First line in a file should be a top-level heading [Context: "#bad.md"]
[00:00:00] Finished 'markdownlint' after 10 ms
Integration with the Grunt build system is similar:
const markdownlint = require("markdownlint");
module.exports = function wrapper(grunt) {
grunt.initConfig({
"markdownlint": {
"example": {
"src": [ "*.md" ]
}
}
});
grunt.registerMultiTask("markdownlint", function task() {
const done = this.async();
markdownlint(
{ "files": this.filesSrc },
function callback(err, result) {
const resultString = err || ((result || "").toString());
if (resultString) {
grunt.fail.warn("\n" + resultString + "\n");
}
done(!err || !resultString);
});
});
};Output:
Running "markdownlint:example" (markdownlint) task
Warning:
bad.md: 3: MD010/no-hard-tabs Hard tabs [Column: 17]
bad.md: 1: MD018/no-missing-space-atx No space after hash on atx style heading [Context: "#bad.md"]
bad.md: 3: MD018/no-missing-space-atx No space after hash on atx style heading [Context: "#This file fails some rules."]
bad.md: 1: MD041/first-line-heading/first-line-h1 First line in a file should be a top-level heading [Context: "#bad.md"]
Use --force to continue.
Fixing
Rules that can be fixed automatically include a fixInfo property which is
outlined in the documentation for custom rules.
To apply those fixes more easily, the applyFixes method in
markdownlint-rule-helpers may
be used.
Browser
markdownlint also works in the browser.
Generate normal and minified scripts with:
npm run build-demoThen reference markdown-it and markdownlint:
<script src="demo/markdown-it.min.js"></script>
<script src="demo/markdownlint-browser.min.js"></script>And call it like so:
const options = {
"strings": {
"content": "Some Markdown to lint."
}
};
const results = window.markdownlint.sync(options).toString();Examples
For ideas how to integrate markdownlint into your workflow, refer to the
following projects or one of the tools in the Related section:
- .NET Documentation (Search repository)
- ally.js (Search repository)
- Apache Airflow (Search repository)
- Boostnote (Search repository)
- CodiMD (Search repository)
- Electron (Search repository)
- ESLint (Search repository)
- Garden React Components (Search repository)
- MDN Web Docs (Search repository)
- MkDocs (Search repository)
- Mocha (Search repository)
- Pi-hole documentation (Search repository)
- Reactable (Search repository)
- V8 (Search repository)
- webhint (Search repository)
- webpack (Search repository)
- WordPress (Search repository)
Contributing
See CONTRIBUTING.md for more information.
Releasing
See ReleaseProcess.md for more information.
History
See CHANGELOG.md.